(1) Field of the Invention
The present invention relates to a 3-substituted polypyrrole and a method for preparing it.
(2) Description of the Prior Art
Polypyrrole has already been shown to become electronically conducting upon oxidation. It is also disclosed in U.S. Pat. No. 3,574,072 that the polymerization of such a polymer can be electrochemically put into practice. The very low value of the oxidation potential of this polypyrrole (E.sub.OX =-0.1 V/SCE) and the good environmental stability of its doped conducting state have raised a large interest, owing to the numerous expectable applications.
Heretofore a large number of works have been devoted to the controlled modification of the polymer properties by varying the dopant characteristics or chemical substitution on the pyrrole monomer. Thus, the substitution on a nitrogen atom has been mainly studied. However, the N-substitution on the polypyrrole results in a steep decrease in the conductivity (&lt;10.sup.-3 S.multidot.cm.sup.-1), and also in an increase of the oxidation potential of the resulting polymer (E.sub.OX &gt;0.6V/SCE).
A recent work describes the functionalization of the polypyrrole by linking ferrocenyl groups to the 3-carbon atom of polypyrrole monomer through an alkyl spacer. Some other works have already pointed out that 3 or 3,4 substitution of polypyrrole monomer has much lower effect than N-substitution on conductivity of the resultant polymer.
In the case of the ferrocenyl-pyrrole monomer, however, the steric hindrance of the substituent does not allow the formation of a continuous homogeneous film. It is known that the above-mentioned problem has been overcome by copolymerizing the substituted pyrrole monomer with unsubstituted pyrrole so as to dilute the ferrocenyl function in the polymer.
On the other hand, it is actually proved by different methods that electrochemical redox process of the polypyrrole include transports in opposite directions inside the polymer, of the two electrolyte species: anions and cations. These two ionic transports can be separated in two different redox systems by using a large-sized electrolyte anion, and the present inventors have recently described the separation of ionic processes in the case of lipophilic polypyrroles.